This invention relates to hydromechanical transmissions, and more particularly to a hydromechanical transmission having rotating cylinder block hydrostatic units on each side of a grounded manifold and related through two planetary gear sets, and a simple clutch for selectively decoupling the pump from the input shaft, and to facilitate ease of external access to the fluid circuit without need of rotating fluid interfaces.
Hydromechanical transmissions of many varieties have been conceived, although only a small number have actually been produced in commercial quantities. Many practical engineering problems must be addressed before a transmission would be considered acceptable for use in a production vehicle. One such problem is dynamic balancing of rotating components. In large machines with low input and output speeds, dynamic balancing is not a critical factor, but in modern passenger cars where the trend is toward higher engine speeds, dynamic balance becomes increasingly important since the unbalance forces are proportional to the square of the rotating velocity. Therefore, all serious engineering designs for such applications must provide for dynamic balance of rotating components, typically by some form of translating counterbalance mass. Complicating the issue, it may be difficult to design a system that is well balanced dynamically over its entire range of operating speeds. Thus, a hydromechanical transmission having components that are inherently dynamically balanced over their full range of operating speeds would be a significant improvement to the technology of hydromechanical transmissions.
The speed and torque range of the transmission must be tailored precisely to the requirements of the driven application and the capabilities of the driver. For example, modern passenger cars generally have an over-drive top gear with a ratio of approximately 0.7:1 and a low gear ratio of approximately 3:1 for a range spread of greater than 4:1. An ideal transmission would make it possible to cover this entire range without the need to shift gears, using hydraulic units large enough to develop the 3:1 low gear multiplication.
Conventional hydrostatic transmissions often include a rotating hydraulic interface by which make-up fluid can be injected into the fluid circuit to make up for fluid losses by leakage. Correct pressure balance is difficult to achieve in such hydraulic interfaces resulting in excessive drag or excessive fluid losses. Removal of such rotating interfaces would provide greatly enhanced efficiencies at final ratio.
The flow rate in conventional hydrostatic transmission is a source of fluid flow losses and noise. A hydromechanical transmission that greatly reduces the fluid flow rate compared to that in conventional hydrostatic CVT""s would offer a significant advantage in efficiency and noise. In particular, if the flow rate at neutral and at final ratio can be reduced to zero regardless of power throughput, and if the maximum flow is only ⅓ that of conventional hydrostatic transmissions of the same displacement, that improvement would be very welcome in the industry.
Many varieties of positive displacement hydrostatic devices exist. Some, such as the vane or ball types offer low cost and/or high reliability at the trade-off of efficiency and/or performance. Ability to use any type of hydrostatic unit that suits the application would be a particular benefit to OEM customers of hydromechanical transmission so that they could tailor the cost and characteristics of the transmission to the application and their customers"" requirements.
Applications for hydromechanical transmission range from large power plants on the order of railroad engines, marine engines and off road loaders and trucks, all the way down to snowmobiles, motorcycles, and even electric scooters. The flexibility to scale up or down to accept any power input and provide an over drive, under drive or 1:1 final ratio without additional external gearing and with any desired efficiency, durability and cost within the trade-off range, affords a manufacturer a simple and inexpensive technical approach to offer a wide range of hydromechanical transmissions in which the transmission design for each customer can easily and quickly be produced for the particular requirements of that customer""s application.
A high input shaft speed often needs to be geared down to produce a moderate pump speed at an elevated torque to better match the input speed to the pump characteristics. Moreover, a clutch or brake mechanism that can be operated to smoothly decouple the input shaft from the pump would be a useful feature to reduce the start-up torque, eliminate lurching on start-up, and provide a soft xe2x80x9claunch feelxe2x80x9d, that is, a smoothly operating, easily controlled acceleration from stand-still without lurching, jerking or lunging.
Accordingly, it is an object of this invention to provide an improved hydromechanical transmission that is simple in design and construction, and is inexpensive to produce, provides high operating efficiencies, and can operate smoothly at high speed substantially free of dynamic imbalance caused by rotating eccentric masses. Another object of the invention is to provide an improved hydromechanical transmission using hydrostatic components that are inherently balanced dynamically over their entire operating speed range. Still another object of the invention is to provide an improved hydromechanical transmission having internal gearing that reduces a high input shaft speed to a speed better suited to a hydraulic pump, and provides an overdrive capability to the wheels. Another still further object of the invention is to provide an improved hydromechanical transmission having an internal clutch mechanism for decoupling the input shaft from the pump to allow the transmission to free-wheel during start-up and other suitable times during operation of the vehicle. Another further object of this invention is to provide an improved hydromechanical transmission that has simple, reliable and leak-free fluid control connections to a stationary manifold block. Yet another object of this invention is to provide an improved hydromechanical transmission having a simple and reliable pump and motor stroke control using a pivoted yoke attached to ground.
These and other objects of the invention are attained in a continuously variable transmission having an operating assembly including an input hydrostatic unit driven by an input shaft, and an output hydrostatic unit hydraulically related to the input hydrostatic unit through a stationary manifold between rotating cylinder blocks of the two units. The input and output hydrostatic units are mechanically related to each other and to the input shaft and an output shaft through at least one variable ratio or epicyclic gearset such as a planetary gearset or differential gearset, all enclosed within a housing. A stationary actuator fixed to the housing is linked to the hydrostatic units for adjusting their displacement, and that actuator is controlled by a control mechanism that causes the actuator to shift to positions that produce the desired transmission ratio. A brake is interposed between two elements in the gearset for selectively and smoothly decoupling and recoupling the input hydrostatic unit from the input shaft to allow the transmission to free-wheel when the brake is operated.